1. Field of the Invention
This invention relates to an apparatus for scanning with a light beam. This invention particularly relates to a light beam scanning apparatus used for reading out or recording an image.
2. Description of the Prior Art
In light beam scanning apparatuses, a light beam emitted by a laser beam source, for example, is reflected by a galvanometer mirror onto a recording medium so as to scan the recording medium in a main scanning direction and, at the same time, the recording medium is moved with respect to the light beam in a sub-scanning direction approximately normal to the main scanning direction by use of, for example, a mechanical feed means, thereby recording or reading out an image. In apparatuses for recording an image, image recording is conducted by modulating the light beam with an image signal by use of, for example, an acousto-optic modulator (AOM) and scanning the recording medium with the modulated light beam. In apparatuses for reading out an image, the recording medium carrying the image recorded thereon is scanned by the light beam and light reflected by the recording medium or light transmitted therethrough is detected to read out the image recorded on the recording medium. Particularly, in a radiation image read-out apparatus for reading out a radiation image stored in a stimulable phosphor sheet comprising a stimulable phosphor, the stimulable phosphor sheet is scanned by the light beam which, as stimulating rays, cause it to emit light in proportion to the radiation energy stored, and the emitted light is photoelectrically detected to read out the radiation image.
In the aforesaid image recording apparatus and the image read-out apparatus, scanning by the light beam should preferably be at a predetermined, constant scanning speed. For example, in the image recording apparatus, the image can be recorded uniformly when the exposure time per picture element is maintained constant. In the case of the image read-out apparatus, the image can be read out uniformly when the read-out time per picture element is constant. Also, the light beam should preferably be scanned over a predetermined scanning range. That is, scanning in the main scanning direction should preferably be started from a predetermined initial position, thereby consistently conducting the image scanning.
However, since the optical system of the light beam scanning apparatus comprises many mechanically moveable elements such as a mirror and a galvanometer, it is not always possible to manually adjust the moveable elements to the optimal condition and to maintain them in the optimal condition. For example, the initial position offset of the scanning beam is greatly affected even by a slight error in the mirror alignment or in the position of the light source. The optical system is also adversely affected by a change in ambient temperature.
Therefore, in order to obtain an image of high quality by scanning at a predetermined, constant scanning speed, a need exists for an apparatus wherein the necessity of manual adjustment is minimized and the initial position offset of the scanning light beam is adjusted automatically.
In view of the above circumstances, a light beam scanning apparatus which will be described below has been proposed in U.S. patent application Ser. No. 543,558 filed on Oct. 19, 1983. The apparatus comprises a scanning means for scanning a light beam in accordance with a first signal, a grid having a plurality of optical gratings arrayed in the scanning direction of the light beam and generating a second signal as the light beam is scanned on the grating array, and a control means for generating the first signal on the basis of the second signal. The control means detects the speed at which the light beam is scanned on the basis of the second signal, adjusts the first signal on the basis of the detected speed, and controls the light beam scanning speed so that the detected speed is maintained equal to a predetermined speed. In this apparatus, it becomes possible to scan the light beam at a predetermined, constant scanning speed. For example, when the light beam scanning apparatus is applied to image recording, it becomes possible to record an image uniformly. When the apparatus is applied to image read-out, it becomes possible to read out an image uniformly.
U.S. patent application Ser. No. 543,558 filed on Oct. 19, 1983 also proposes a light beam scanning apparatus comprising the aforesaid means wherein the control means detects the time required for the light beam to scan from a scanning start point to the end of the grating array on the basis of the second signal, and adjusts the initial level of the first signal on the basis of the detected time, thereby controlling the start point of scanning of the light beam. In this case, it becomes possible to automatically control any change in initial position offset amount of the scanning light beam which may be caused, for example, by an error due to manual adjustment of optical members or an error due to a change in ambient temperature. Thus scanning by the light beam can be adjusted to and maintained in the optimal condition.
However, in the aforesaid light beam scanning apparatus, the number of parts becomes large. Therefore, the apparatus becomes large and difficult to handle. Further, the manufacturing cost is high since the grid is expensive.
Also, the aforesaid light beam scanning apparatus is disadvantageous in that, when ambient dust sticks to the surface of the grid, it becomes impossible to obtain a consistent synchronizing signal for controlling the light beam scanning from the grid.